Documentation: warn prominently against merging with dirty trees

We do this for both git-merge and git-pull, so as to hopefully alert
(over)users of git-pull to the issue.
Signed-off-by: NThomas Rast <trast@student.ethz.ch>

Documentation/git-merge.txt

@@ -22,6 +22,10 @@ The second syntax (<msg> `HEAD` <commit>...) is supported for
 historical reasons.  Do not use it from the command line or in
 new scripts.  It is the same as `git merge -m <msg> <commit>...`.
 
+*Warning*: Running 'git-merge' with uncommitted changes is
+discouraged: while possible, it leaves you in a state that is hard to
+back out of in the case of a conflict.
+
 
 OPTIONS
 -------

Documentation/git-pull.txt

@@ -24,6 +24,10 @@ when merging local branches into the current branch.
 Also note that options meant for 'git-pull' itself and underlying
 'git-merge' must be given before the options meant for 'git-fetch'.
 
+*Warning*: Running 'git-pull' (actually, the underlying 'git-merge')
+with uncommitted changes is discouraged: while possible, it leaves you
+in a state that is hard to back out of in the case of a conflict.
+
 OPTIONS
 -------